Targeted anti-cancer therapeutics are designed to reduce nonspecific toxicities and increase efficacy relative to conventional cancer chemotherapy. This approach is embodied by the powerful targeting ability of monoclonal antibodies to specifically deliver highly potent, conjugated small molecule therapeutics to a cancer cell. In an attempt to address the issue of toxicity, chemotherapeutic agents (drugs) have been coupled to targeting molecules such as antibodies or protein receptor ligands that bind with a high degree of specificity to tumor cells to form compounds referred to as antibody-drug conjugates (ADC) or immunoconjugates. Immunoconjugates in theory should be less toxic because they direct the cytotoxic drug to tumors that overexpress the particular cell surface antigen or receptor. This strategy has met limited success in part because cytotoxic drugs tend to be inactive or less active when conjugated to large antibodies or protein receptor ligands. Promising advancements with immunoconjugates has seen cytotoxic drugs linked to antibodies through a linker that is cleaved at the tumor site or inside tumor cells.
A chemical solution to targeted delivery of cytotoxic or cytostatic drugs conjugated to cell-specific ligands is the “self-immolative linker”, PABC or PAB (para-aminobenzyloxycarbonyl), attaching the drug moiety to the ligand in the conjugate (Carl et al (1981) J. Med. Chem. 24:479-480; Chakravarty et al (1983) J. Med. Chem. 26:638-644). The PAB linker unit is also referred to as an electronic cascade spacer. The amide bond linking the carboxy terminus of a peptide unit and the para-aminobenzyl of PAB may be a substrate and cleavable by certain proteases. The aromatic amine becomes electron-donating and initiates an electronic cascade that leads to the expulsion of the leaving group, which releases the free drug after elimination of carbon dioxide (de Groot, et al (2001) Journal of Organic Chemistry 66(26):8815-8830). Cathepsin B is a ubiquitous cysteine protease. It is an intracellular enzyme, except in pathological conditions, such as metastatic tumors (Sinha et al (2001) Prostate 49:172-184) or rheumatoid arthritis (Hashimoto et al (2001) Biochem. Biophys. Res. Commun. 283:334-339). Therefore, conjugates produced with cathepsin B-cleavable linkers are likely to be stable in circulation. Upon cleavage of a peptide bond adjacent to the PABC, i.e. by an intracellular enzyme, the drug is released from the ligand whereby no remaining portion of the linker is bound (de Groot, et al (2002) Molecular Cancer Therapeutics 1(11):901-911; de Groot, et al (1999) J. Med. Chem. 42(25):5277-5283).
Linkers containing the para-aminobenzyloxycarbonyl (PAB or PABC) unit, in conjunction with a peptide unit, have been developed with a “self-immolating” or “self-immolative” mechanism of 1,6 elimination and fragmentation under enzymatic, hydrolytic, or other metabolic conditions to release a drug moiety from a targeting ligand, such as an antibody (U.S. Pat. No. 6,214,345; U.S. 20030130189; U.S. 20030096743; U.S. Pat. No. 6,759,509; U.S. 20040052793; U.S. Pat. No. 6,218,519; U.S. Pat. No. 6,835,807; U.S. Pat. No. 6,268,488; U.S. 20040018194; WO98/13059; U.S. 20040052793; U.S. Pat. No. 6,677,435; U.S. Pat. No. 5,621,002; U.S. 20040121940; WO2004/032828). The 2-nitroimidazol-5-ylmethyl group has been reported as a fragmenting prodrug unit (Hay et al. (1999) Bioorg. Med. Chem. Lett. 9:2237). For the use of the PAB unit in prodrugs and conjugates, see also: Walker, et al (2004) Bioorganic & Medicinal Chemistry Letters 14(16):4323-4327; Devy, et al (2004) FASEB Journal 18(3):565-567, 10.1096/fj.03-0462fje; Francisco, et al Blood (2003) 102(4):1458-1465; Doronina, et al (2003) Nature Biotechnology 21(7):778-784; King, et al (2002) Journal of Medicinal Chemistry 45(19):4336-4343; Dubowchik, et al (2002) Bioconjugate Chemistry 13(4):855-869; Dubowchik, et al (2002) Bioorganic & Medicinal Chemistry Letters 12(11):1529-1532.
Limitations of the PAB type self-immolating linkers are the propensity to cause poor solubility and aggregation of the conjugates. In addition, some PAB-containing conjugates may not be suitable substrates for certain cleaving enzymes or cleave too slowly to achieve efficacy. It would be desirable to improve the properties of drug-ligand conjugates by optimizing the structure of a self-immolative linker.